why is the name of a color used after a subject in a spanish sentence
why is it " fresa rojo" instead of "rojo fresa"
Basically because Spanish is structured differently than English. For most (but not all) cases the descriptive adjective will follow the noun rather than precede it. Here are a couple of links that you might find useful:
Try the free video lessons and have a look at the reference section while you are here. There is a ton of useful information here for the taking.
Do you mean after a noun?
I'm not sure that it does always follow the noun, but it does in the majority of the cases. The reason is that colors are considered descriptive adjectives.